The present invention relates generally to solar cells and/or other light-to-electrical energy transducers; and, more particularly, to solar cells--especially, concentrator solar cells--and to methods of manufacture thereof, characterized in that the cells have a multiplicity of closely spaced, fine-line--e.g., on the order of less than about 1.0 mils and, preferably, less than about 0.7 mils--electrodes formed on the surface thereof by a process wherein successive thin layers of, for example, titanium, palladium (each on the order of 500 A in thickness), and silver (on the order of 1000 A in thickness) are first deposited on the contact areas of the cell by means of a conventional vacuum deposition process and, thereafter, the thickness of the fine-line electrodes is significantly increased to a thickness at least as great as the width of the fine-lines and, preferably, to at least on the order of 0.7 mils, by an electroplating process (preferably using a silver cyanide plating process) while employing a relatively thick exposed and developed photoresist mask so as to form electrodes which, despite having a thickness at least as great as the width thereof and, preferably, at least 0.7 mils, or greater, nevertheless are characterized by their sharp vertical edge profiles and which are essentially devoid of lateral "spreading," a phenomenon commonly encountered with conventional electroplating processes and which tends to mask the photoactive cell regions on either side of the fine-line electrodes, thereby reducing cell efficiency.
One of the more perplexing problems faced by designers, manufacturers and users of light-to-electrical energy transducers such as solar cells and concentrator solar cells has, for a number of years, been the need to improve both the light energy collection efficiency and the conversion efficiency of light-to-electrical energy transducers. In this connection, it has long been recognized that light reflected from the face of a solar cell is a principal source of poor light collection efficiency, and many efforts have been made, and are continuing to be made, to solve this problem. Initially, such efforts were primarily directed towards providing a thin, non-reflective, transparent barrier layer; but, such non-reflective barrier layers, of and by themselves, have not provided a satisfactory solution to the problems. More recently, it has been proposed that the photoactive regions of the photocell substrate be "texturized" by the use of a texturizing etchant so as to form a surface characterized by randomly located irregularities (commonly pyramidal in shape) defining light absorptive surfaces having reflecting facets to increase collection efficiency. Those interested in a more detailed description of the foregoing problems and solutions thereto are referred to the aforesaid co-pending application of Billy J. Stanbery, Ser. No. 169,790 filed July 17, 1980, and assigned to the assignee of the present invention.
The rapidly advancing state of the technology relating to concentrator solar cells and the like which has led to attainment of ever increasing light collection efficiencies and light-to-electrical energy conversion efficiencies has increased the demand for reliable and effective solar cell electrode arrangements characterized by their ability to effectively collect the increased electrical current generated in such cells. Of course, one apparent way to improve the current conduction capacities of solar cells is simply to increase the surface area of the electrodes formed on the cells; but, conversely, increasing the surface area of that portion of the cell which is covered by electrode materials also serves to reduce the photoactive surface area of the cell, thereby reducing both light collection efficiency and light-to-electrical energy conversion efficiency.
Various techniques have been used and are well-known in the art for forming electrodes on the surface of a solar cell. These include, merely by way of example, conventional vacuum deposition techniques, ion sputtering techniques and/or electroplating techniques. Generally, however, the formation of electrodes with the use of vacuum deposition techniques and/or ion sputtering techniques is limited to the application of relatively thin electrodes which are commonly measured in angstroms (A). Electroplating techniques, on the other hand, permit the application of considerably thicker electrodes. Unfortunately, however, electroplated electrodes tend to spread laterally as the thickness of the electrode increases; and, typically, such electroplated lines tend to spread to a width on the order of twice the thickness of the electroplated electrode. For example, where one is attempting to form an electrode on the order of 1.0 mils in thickness, a conventional electroplating process tends to actually produce an electrode which is on the order of 2.0 mils in width. With concentrator solar cells of the type here under consideration, it is desirable that the electrodes have an extremely fine-line geometry, on the order of less than about 1.0 mils and, preferably, less than about 0.7 mils in width; and, consequently, when one attempts to form such fine-line electrodes with thicknesses of up to, for example, 0.7 mils to 1.0 mils in thickness, the "spreading" phenomenon actually produces a resultant electrode which will range from approximately 1.5 mils in width to 2.0 mils or greater--i.e., thereby representing a significant increase in the area of the photocell which is effectively masked by electrode materials by an amount on the order of from 300% to 500%. This, of course, results in a significant reduction of photoactive material which can be exposed to incident radiation.
As indicated above, the prior art is replete with a wide range of techniques for applying electrodes to the contact areas of light transducers. One early disclosure is that contained in Falkenthal U.S. Pat. No. 2,034,334 which describes a very primitive method for applying electrodes. Subsequently, electrodes have generally been applied in the form of grid lines in the manner disclosed in Hansell U.S. Pat. No. 2,310,365 and in Ross et al. U.S. Pat. No. 3,411,952.
Iles et al. U.S. Pat. No. 3,361,594 represents an early patent disclosure wherein a contact pattern is formed on the upper surface of the cell either by the use of a conventional printing process or by the use of a suitable photoresist mask. The cell is then etched with hydrofluoric acid or the like to form openings in the photoresist mask or the like which are subsequently filled with metal to form relatively thin electrodes.
King U.S. Pat. No. 4,086,102 discloses a process in which the surface of the cell is masked and an ion beam sputtering process is employed to form relatively thin electrodes through the mask. Revesz et al. U.S. Pat. No. 3,904,453 employs a photoresist mask and etching techniques to remove metallic and oxide layers from the substrate beneath the developed openings in the mask. Thereafter, a thin metal film is deposited through the openings in the mask to form the electrodes. Revesz et al. and Dyer et al. U.S. Pat. No. 4,115,120 also disclose various conventional lift-off methods to pattern thin metal evaporated coatings and to facilitate removal of the masks employed.
Other patents of general interest include Lindmayer et al. U.S. Pat. No. 3,982,964, and Yerkes et al. U.S. Pat. Nos. 4,105,471 and 4,165,241.
In general, however, the various prior art approaches which have been, and are conventionally being used and which are described in the patents noted above, are concerned primarily with the application of relatively thin electrodes which may, or may not, comprise fine-line electrodes--i.e., electrodes on the order of less than 1.0 mils in width--but, which are generally quite thin, being measured in angstroms rather than in mils. In such thin electrodes, the problem of spreading of the electrode materials as the electrode is formed has simply not been faced.